We humans differ from "lower" mammals in several ways: the complexity of our large brains, our upright posture, our manual dexterity and our capacity for articulate speech. Our intelligence, the product of our complex brain, affords many advantages, including our ability to reason, imagine, theorize, create, communicate and plan, among many other cognitive functions. This brain power also comes with a price.
Relative to other mammals, we humans undergo slow physical development and remain highly dependent upon our parents for a decade or more. While this provides a wealth of vital experience for our developing brains, it puts us at risk for an early death; of course, this was especially true before the era of modern technology and medicine, when children were exposed to a wide variety of natural threats. Furthermore, our slow physical development delays sexual maturity, reducing the capacity of human populations to recover from catastrophic events.
On a more personal level, intelligence produces a source of stress that is not experienced by other mammals. Humans must deal with the effects of regret, guilt, fear, criticism and anticipation. The greatest price, of course, is the knowledge that life is transient and that we, like all other creatures, will die; it is this knowledge that has molded human culture from its earliest days, spawning beliefs that serve to douse our fear.